The present disclosure relates generally to neutron-induced gamma-ray spectroscopy and, more particularly, to techniques for determining absolute elemental concentrations from neutron-induced gamma-ray spectroscopy.
Using nuclear downhole tools, the elemental concentration of a subterranean formation may be determined using a variety of techniques. An indirect determination of formation lithology may be obtained using information from density and photoelectric effect (PEF) measurements from gamma-ray scattering in the formation. A direct detection of formation elements may be obtained by detecting neutron-induced gamma-rays. Neutron-induced gamma-rays may be created when a neutron source emits neutrons into a formation, which may interact with formation elements through inelastic scattering, high-energy nuclear reactions, or neutron capture.
Gamma-rays emitted in inelastic scattering events (“inelastic gamma-rays”) or neutron capture events (“neutron capture gamma-rays”) may have characteristic energies that, based on various spectroscopy techniques, may identify the particular isotopes that emitted the gamma-rays. Techniques involving inelastic spectroscopy interpretation may be based on ratios of elemental yields attributable to inelastic gamma-rays of various characteristic energies. Most notably, the ratio of the number of detected gamma-rays due to carbon versus those due to oxygen (“C/O ratio”) has been used to estimate formation oil saturation. An advantage of using a ratio is that some instrumental effects, such as variable neutron output and a number of environmental effects, will cancel out. A disadvantage of using a ratio is that it is usually more difficult to interpret. For the simple case of estimating oil saturation in a water-filled borehole, the C/O ratio may be complicated by gamma-rays attributable to oxygen from borehole fluid and the cement annulus, whereas all gamma-rays attributable to carbon would derive from the formation.
Similar techniques involving neutron capture spectroscopy may involve collecting and analyzing neutron gamma ray energy spectra. Elements typically included in a neutron capture spectrum may include Si, Ca, Fe, S, Ti, Gd, H, Cl, and others, and sometimes Al, Na, Mg, Mn, Ni, and other minor or trace elements. However, the elemental concentrations determined using such techniques may also generally identify only relative concentrations of formation elements, unless an absolute concentration of a formation element is already known or properly estimated.
Certain other techniques for estimating absolute elemental concentrations in a formation may involve oxide closure normalization of spectroscopy log data, or may involve supplementing spectroscopy log data with activation and/or natural gamma-ray measurements. However, closure normalization may depend on accurate associations for unmeasured elements, which may change depending on the exact assembly of formation elements. Additionally, closure normalization may depend on using all elements that may influence the spectrum (except K and Al), some of which may not be as precisely determined as others. The use of activation and/or natural gamma-ray measurements may also have various disadvantages. In particular, such measurements may often involve highly complex tools and long measurement times.